Growing Old With You
by skygirl55
Summary: What if you didn't age until you met your soulmate? Caskett Soulmate AU One-shot.


**Growing Old With You**

What if you didn't age until you met your soulmate? Caskett Soulmate AU from a Tumblr prompt

* * *

Sitting at his kitchen island taking a gentle sip of his still scalding hot coffee, Richard Castle decided that day was going to be a good day: the sun was shining, birds were chirping, the warmth of spring was finally creeping into the chilly winter air. As the weather was beginning to grow steadily warmer, he'd spent the weekend out at his house in the Hamptons making sure everything was ready for the summer months. Plus, he'd snuck in a little bit of uninterrupted writing time, which had been surprisingly productive. Now that he was back in the city, Castle was eager to return to his regular routine of visiting the always-lovely Detective Beckett, but first he would enjoy his breakfast casually.

Fifteen minutes later, a whistle on his lips and a spring in his step, Castle crossed through the living area and his office to enter his bedroom and adjoining bath. He showered efficiently before wrapping a towel around his waist and stepping in front of the anti-fog mirror to examine the stubble on his chin. Leaning close to the glass he tilted his head from side to side, considering. As he had never been overly hairy, the three days' worth of growth was far from excessive, but he did prefer to be clean-shaven as a rule, particularly when shadowing his partner and doing his best to appear professional.

Just as he reached for his razor and shaving cream, Castle caught sight of something odd on the left side of his chin. Something glinted in the bathroom light at the edge of his jaw directly below the left corner of his mouth. It almost seemed as though a droplet of water remained on his flesh, but his face was completely dry. Leaning a bit closer to the mirror, he skimmed his index finger over the spot and realized with a mixture of delight and shock that the reason the light was reflecting differently was not due to water, but the change in hair. He had a white hair on his chin! A single white hair in his otherwise brown beard.

He had a white hair!

Immediately flustered, Castle jumped back from the mirror so aggressively the towel slipped from around his waist. He jerked back and forth in front of the mirror a few times, unsure of where to go next, but then an idea hit him. He wanted to be sure—absolutely certain—of his discovery and he did not feel his regular bathroom mirror could provide enough detail, but the magnifying mirror his mother used to apply her stage makeup certainly could.

Castle dashed from the bathroom, forgetting he was completely nude until he was halfway across the bedroom. Groaning, he turned back, grabbed a pair of boxers and tugged them on before hurrying up the stairs to the bathroom his mother and daughter shared. He flicked on the light and grabbed the round mirror from the counter. Angling his chin towards the light, he adjusted the angle of the mirror several times before he caught a glimpse of what he was searching for. There was a white hair on his chin; he was sure of it.

"Yes!" Castle cheered aloud, punching the air as he dropped the mirror to the counter. He was aging— _finally aging_ —and considered in the prior month he had only kissed one person…well, there was no question in his mind as to the identity of his soulmate and he could not have been happier about it.

* * *

From the day he turned eighteen, Richard Castle desired one thing above all else: finding and kissing his soulmate. Not just finding her—that wasn't quite enough. But the kiss—the kiss! That very first kiss would be the most important marker in his life, for it would be the day he no longer looked like a late-teen, no matter his chronological age, and he began to age into a real adult. While others might have bene horrified at the prospect, he wasn't; Castle couldn't wait.

Centuries before his birth—almost from the dawn of recorded history—humans were blessed with the ability to discover soulmates and grow old together with their one true love. Both males and females aged regularly until the age of eighteen and then existed in a suspected state until meeting their soulmate. Then, they aged at an increased pace for a few years before that rate leveled out and they settled into middle-age—together. Though there was debate about what triggered the aging—some believed it was at first sight while others believed it wasn't until love was shared—the general consensus had settled on the fact that the first shared kiss was the real trigger.

Despite the long odds, more people than not found their soulmate—around seventy percent, in fact. For those who were unlucky, they never looked a day older than eighteen, even if they were four times that age. While some could certainly argue that, in a way, was its own form of luck, Castle strongly disagreed. He welcomed getting older as long as it was with an incredible woman with whom he was destined to spend his life.

Though Castle faced two failed marriages and over two decades of waiting, his faith never waivered. Not one of his family members—his mother included—had ever found their soulmate, but he believed he was unquestionably the one to break that streak. He would find her; he only needed to be patient.

When Castle first met Kate Beckett, he was completely smitten with her. She was fascinating in every conceivable way. He wanted to soak up all of her that he could—her likes and dislikes, what drove her, what she felt passionate about, and, perhaps most importantly, why she had the most devastatingly sad eyes. With patience and persistence, he was slowly able to chip away at her walls and find answers to his questions that only led to more questions. She inspired him to be better in so many ways. He wanted to be better for he felt likely he was only truly himself—the man he was meant to be—in her reflection.

Even with those strong feelings—most he'd never felt before—it took Castle nearly two years to think of Kate Beckett as his potential soulmate. When he was stuck outside on the street watching her apartment explode into a ball of flames—that's when it hit him. His heart throbbed and his gut twisted as he feared he might have listened to her take her last breath during their phone call. He cared for her as a person, and as a friend, but in that moment it was more; he knew his future happiness was tied to her.

Despite this knowledge, Castle didn't act on his feelings for he knew that they weren't yet in the right place. True, he could have grabbed her and kissed her and, whether or not she slapped him afterwards, the aging process would start for both of them, but he didn't want it that way; he didn't want it forced. He had waited two decades; he could wait a little longer for the right moment; the perfect moment.

Three weeks earlier their moment had arrived, and while it might not have been perfect, it was certainly incredible.

Under the ruse of an undercover disguise, Castle had grabbed Kate and kissed her like his life depended on it. Despite the fact that she hadn't known the kiss was coming, Kate kissed him back. She _kissed him back_ —nipping at his bottom lip, crushing her body against his, and—god help him—moaning against his mouth. It was, without question, one of the best moments of his life. The morning following their lip-lock Castle had searched his scalp quite thoroughly for any hairs that were not purely brown, but of course his search had been for naught. He wasn't sure why he excepted the aging process to start instantly—perhaps even retroactively—but of course it had not. Over the next week he'd made daily scalp-checks, but then had grown frustrated searching for something that might not even exist. Yes, he believed Kate to be his soulmate, but that didn't mean that she was for certain and he knew he had but one choice, even though he didn't like it at all; he had to be patient.

Finally—finally—he needed to be patient no longer, for he had official confirmation: Kate Beckett was his soulmate.

Elated and partially delirious with joy, Castle returned to the main floor of his apartment and paced back and forth unsure of his next course of action. He wanted to throw on clothes, race out the door, and show up at the Twelfth precinct screaming like a lunatic and pointing at his chin, but he knew he could not do that for Kate's sake and his own. This was Kate Beckett, the woman with a security system around her heart that rivaled Fort Knox. He didn't need to tell her about their status; she would see the hairs on herself sooner than later. Though he wanted to tell her, his gut told him it was better for her to discover on her own. After all, she had just broken up with her boyfriend of several months (a factor he discovered barley a week prior); she wasn't in a mental or emotional state to jump into another long-term relationship yet and he was fine with that. For Kate Beckett, he would wait however long it took, because he knew, without any uncertainty, they were bound to be extraordinary.

* * *

Sitting in "his" chair beside Beckett's desk at the Twelfth, Castle trapped his tongue between his teeth as he swiped his thumb back and forth across the screen of his phone trying to get a few angry, colorful birds to fly in the direction he wanted them to. Beside him, his detective partner was filling out paperwork to request a warrant. Then, they would be going to the morgue to see if there was an official COD for their victim yet. Until then, he had no problem passing the time beside his partner.

"Whoa Castle!"

The writer looked up from his phone to see Detective Ryan passing by, folder in hand, his eyes wide. Without much thought Castle asked, "What's up?"

Looking a little shell-shocked, Ryan pointed his index finger towards Castle's head. "You…did you know you have white hairs on top of your head?"

Immediately feeling his face flush since they were in such close proximity to Beckett, who clearly would have heard Ryan's question if she was paying any attention at all to her surroundings, he stammered, "Oh, ah…where—where are they?"

Ryan stepped forward and pressed his index finger to Castle's scalp at a spot about four inches back from his hairline very near the crown of his head. "Here."

Castle tucked his phone into his pocket and nodded. Though he had been casually checking his scalp every time he washed his hair, he had yet to notice anything obvious. Three weeks had passed since he first found the white hairs on his beard, so he was not shocked to learn some had sprouted atop his head as well. "Oh I guess I didn't see those yet, but I, ah, have a few in my beard so I figured they'd show up eventually," he said, trying to pass off the incident as casual by adding a light laugh at the end of his statement.

Clearing thinking the situation as anything but casual, Ryan merely blinked for several moments. Then he said, "Wha…you didn't tell us you were seeing anyone right now…"

"That's because I'm not."

Jumping to conclusions, Ryan gasped, "Oh my god—was your soulmate a one-night-stand?!"

Detective Esposito chose that precise moment to walk through the area and picked up on a key word in Ryan's exclamation. Looking between writer and detective, Espo asked, "What? Castle found his soulmate?"

"Yeah look at his hair!"

Able to see the top of the writer's head with ease since he was sitting, Espo peered at the spot Ryan pointed to and then laughed. "Ha! You're going to get so gray!" As neither of the male detectives had yet to find their matches, they both looked as young as they had in high school—not that it mattered. At least half of the senior staff members of the Twelfth had found their mates, so Castle's graying hair would not look that dramatically out of place.

Before the writer could attempt to explain himself, the still-shocked Ryan continued with, "Seriously—was it a one-night-stand? Maybe we can help you find her…"

"Guys, guys it's okay." Castle assured them. "I know who she is I just…it's just complicated right now…" Boy, that was an understatement, but he felt it was also a believable explanation—and one the boys wouldn't question, which was good, because he didn't want to share any more details lest they guess the identity of his mate.

Sounding more than a little hurt, Ryan frowned and said, "But you didn't tell us…"

"Maybe because he didn't want to be interrogated," Kate said with a purposeful tone, speaking for the first time since their conversation began.

Castle actually jumped slightly at the sound of her voice, having momentarily forgotten she was still there, listening to them. Venturing a glance over to her, she saw she stared down her partners and didn't meet his eye. He wasn't immediately sure if that was a good sign or a bad one, so he tried his best to brush the incident off as no big deal. "Guys—it's fine. I promise I will tell you all about it when the time is right."

Accepting this, Espo nodded and walked away. His partner lingered, giving the writer a guilty frown as he apologized. "Sorry Castle."

Castle smiled at him reassuringly. "No worries, Ryan." After he left, Castle turned back to Kate and saw she was once again focused on her warrant request. He didn't want to interrupt her, but yet felt he had to say something now that Ryan had dumped the proverbial cat out of its bag.

Five minutes later, her request submitted, the duo walked towards the elevator to go down to the parking level where her squad car awaited them. Only once they were alone inside the descending car did Castle speak again. He didn't want to force her into a conversation she wasn't yet ready for, but, as per his personality, he simply couldn't let it go. "For the record," he began softly, "I didn't tell Ryan, Espo, or anyone else because I didn't think you were ready to talk about it yet."

She gazed at him for fifteen seconds before giving him a subtle nod. "Thank you."

He smiled in the reflection of her honey-brown eyes. "You're welcome." They were silent a moment before he added, "While we're on the subject, though—I'm curious. Where did you notice your first white hair?"

She let out a slight laugh and pointed her index finger towards the middle of her forehead. "Grew out of my hairline right here; dead center. I pulled it out the morning I found it."

Thinking he might have done the same if his first white hair was in that obvious of a position—particularly since they had yet to go "public"—Castle merely nodded. "Oh."

"Not because I was mad or upset," she added quickly. "I just…I wasn't ready for people to know yet—especially since I'd just ended things with Josh."

"Of course; I understand."

When the elevator came to a stop at the underground parking level, the doors opened and the duo stepped out. Kate walked two steps to her left and then stopped, turned around, and gazed at him looking the slightest bit uncertain. "Um…Castle?"

"Yeah."

"I just wanted to say: I'm not quite ready for us yet. That is: I'm still trying to wrap my head around the fact that it's you. Even…even though we've been in each other's lives for about three years now I still…I guess I never let myself believe that someone like you could really be the one for me."

Unsure of her meaning, he arched his eyebrow and echoed, "Someone like me?"

She gave her head a gentle shake and stepped forward, her expression turning soft. "I don't mean it in a bad way. I just mean…someone who challenged me and made me want to be better while at the same time making me laugh and encouraging me to have fun. I never thought those two types of people could be one in the same, but I'm glad that they are. I…I'm glad that it's you."

Instantly a grin so wide it made his cheeks ache formed on Castle's face, but he didn't care one bit. His heart felt like it was going to fly right up out of his throat and it was the best feeling he'd ever had. "I am too, Kate."

She nodded and then continued to lead the way towards her parked squad car. Castle scurried to keep up with her so they could walk side by side. Just as they sat down in the car, he turned his body towards hers and asked, "Did…did you maybe want to have dinner sometime this week? As friends, I mean."

She smiled. "I'd like that."

"Me too. I mean—great. It's…a friend date." His heart still fluttering in his chest Castle reached for his seatbelt, fumbled with it for a moment, and then finally latched it correctly. When he looked up, Kate gazed at him with a high amount of amusement, but he didn't even feel that embarrassed, because he already knew they were going to have an incredible future together.

"You know," she said as the engine turned over, "we're still going to have to act professionally here at work."

"Beckett, please." He waved his hand flippantly and leaned back in his seat. "I'm always professional."

She arched a skeptical eyebrow. "Always?"

"Yes, always."

"Really? Because I think I can name about a dozen times when you were not."

His tongue clicked with annoyance. "A dozen—no way. I bet you can't name more than five."

Her eyebrow quirked. "Winner pays for dinner?"

He nodded. "Deal."

A slow smile crossed her face as she slowly eased the car out of its parking space. "Then get your wallet ready, Castle. Number one—the first day we met…."

* * *

 **Thanks for reading!**


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